Estimating the total number of dispensable M. genitalium orthologs in M. pneumoniae. Among the 150 putative gene disruptions in M. pneumoniae, 57 are in genes that have orthologs in the M. genitalium genome, and 93 are in M. pneumoniae–specific genes. We have identified disruptive insertions in 47% (93/197) of the M. pneumoniae–specific genes. It is then a reasonable assumption that 47% of the dispensable M. pneumoniae genes common to both genomes have also been disrupted. This leads to an estimate that ∼121 M. genitalium orthologs are dispensable and ∼318 genes in total (121 + 197) are dispensable in M. pneumoniae. An analogous calculation using only those genes that have been hit more than once leads to an estimate of ∼108 dispensable M. genitalium orthologs.